1. Field of the Invention
This invention relates generally to thin film magnetic head fabrication processes and, more specifically, to a sacrificial layer process for fabricating a narrow inductive head having a submicron track width.
2. Discussion of the Related Art
Thin film magnetic read/write heads are used for reading and writing magnetically coded data stored on a magnetic storage medium such as a magnetic disk or magnetic tape. There is a continuing strongly-felt need for increasing the data storage density in such media. Most efforts to increase magnetic data storage density involve techniques for increasing the areal bit density in the magnetic medium.
In rotating magnetic disk drives, the areal density is equivalent to the product of the number of flux reversals per millimeter and the number to tracks available per millimeter of disk radius. Thus, high areal data storage density requires recording heads with high linear resolution and narrow track-width. The linear resolution of a two-pole inductive head is related to the gap between the pole-tips at the air bearing surface (ABS). In the present art, submicron gaps are commonly available. Recent improvements in magnetoresistive (MR) sensor fabrication have led to development of the dual element head, which combines MR read and inductive write elements. This dual element approach solves the low read-back signal sensitivity problem associated with narrow inductive heads. Thus, increased linear recording is now obtainable without incurring unnecessary penalties in lost signal sensitivity.
In pushing the areal density limit in magnetic recording using the dual MR-inductive element approach, the problems associated with fabricating narrow-track inductive write heads are now more limiting than the problems associated with fabricating narrow-track MR read heads. Experimental and mathematical modeling results confirm that further substantial increases in areal recording density must come from reductions in track width rather than from increases in linear flux transition densities in the recording media.
The major barrier to narrower track widths imposed by conventional thin film inductive head fabrication techniques is the topographical variation confronted when defining the upper pole-tip width. Because of this varied topography, conventional thin film techniques require the narrow upper pole-tip to be deposited at the bottom of a 15-18 micron photoresist groove. Unreliable results are well-known for attempted deposition of a layer width of one to three microns at the bottom of a 15-20 micron groove depth. Several new pole-tip fabrication approaches have been proposed to address this problem.
One such is the "staggered" pole-tip magnetic head design, which is inherently different from the thin film inductive head designs of the earlier art. As used herein, "staggered" pole-tips means pole-tips that are overlapped to form a narrow gap in the overlap region. The gap width is often much narrower than either of the overlapping pole-tips. Po-Kang Wang, et al ("Thin Film Head With Staggered Pole-tips", IEEE Transactions on Magnetics, Vol. 27, No. 6, November 1991, pp. 4710-4712) propose a cost-effective longitudinal staggered pole-tip configuration and an alternative transverse staggered pole-tip configuration both suitable for submicron track-widths. Wang, et al report that both configurations should be suitable for narrow track applications but they found that conventional process variations provided low-yield fabrication of staggered pole-tips in the transverse configuration and they were unable to reliably fabricate any submicron track-widths in the longitudinal configuration. Because of the particular problems imposed by the conventional process employed, a slanted and uneven ABS gap was obtained (e.g., FIGS. 2 and 3).
Other practitioners avoid the track-width limitations of variable topography by using a "stitched" upper pole concept that defines the upper pole-tip immediately following the deposition of the insulating gap defining material. As used herein, a "stitched" pole is a pole formed in two or more separate steps, such as where a pole-tip is first deposited and a pole-yoke later deposited and joined (stitched) to the existing pole-tip. For instance, T. Kawabe, et al ("Fabrication of Thin Film Inductive Heads With Top Core Separated Structure", IEEE Transactions on Magnetics, Vol. 24, No. 6, November 1991, pp. 4936-4938) discusses a pole-tip stitching method where the upper pole-tip is formed at an early point in the fabrication process when a shallow photoresist layer can still be used as a pole-tip mask. However, Kawabe, et al neither teach nor suggest a method for ensuring proper formation of the junction between the pole-tip and the pole-yoke portions of the upper pole.
The "self-aligning" pole-tip fabrication methods known in the art may also result in improved yield of narrow-track heads, as can be understood with reference to three typical disclosures. In U.S. Pat. No. 4,436,593, John R. Osborne, et al disclose a self-aligned pole-tip fabrication method for precisely aligning the pole-tips of a thin film magnetic head. Osborne, et al teach the use of the upper pole as a mask for etching the lower pole, thereby insuring pole-tip alignment at a narrow gap region. In U.S. Pat. No. 4,992,901, Beat Keel, et al teach a self-aligning method for fabricating magnetic poles using a sacrificial mask layer. Keel, et al use a sacrificial mask to protect the upper pole-tip while removing the exposed non-aligned portions of the gap-forming material and lower pole-tip. They first deposit the upper pole and sacrificial mask layers in a hole cut away from a thick photoresist mask layer and then use these built-up elements as a mask for etching away the excess in the lower layers. In U.S. Pat. No. 4,837,924, Jean-Pierre Lazzari teahes a process for the production of a planar thin film magnetic head where a recessed slot is etched in a substrate and filled with a magnetic film. Thus, like Keel et al, Lazzari teaches a process of first etching a defined hole in a substrate, next filling the hole with material, and finally using the material thus deposited as a mask for further etching in the layers below the deposited material. This procedure is now well-known and generally accepted as the preferred method for depositing narrow layers of thin film materials.
Although the stitched pole concept, the staggered pole concept and the self-aligning pole-tip concept all appear useful for fabricating inductive write heads having track widths under three microns, existing thin film fabrication procedures do not provide acceptable yields. For instance, submicron track widths can be fabricated using the staggered head approach without encountering lithographic resolution or aspect ratio problems but founder on problems of planarization control. Both staggered head and stitched head designs require well-controlled planarization processes and the staggered head requires a method for adding a precise step on each pole-tip to reduce side-writing effects.
Unfortunately, conventional methods do not ensure uniform planar gap regions in inductive heads having track widths under three microns. FIG. 2 shows an outline of a photomicrograph of the ABS aspect of a 2.5 micron staggered pole head. Note the extreme curvature and bleeding effects where the two pole-tips P.sub.1 and P.sub.2 abut the nonmagnetic insulating wall 10 defining the stepped edge of the staggered pole recording (flux-sensing) gap 12. Note the skewing of recording gap 12 with respect to the gap centerline 14. Similarly, FIG. 3 shows the ABS aspect view of a 0.5 micron staggered inductive head having a gap 16 that is almost entirely tilted with respect to the gap centerline 18 because of similar edge deposition effects at the insulating wall 20. There is a clearly felt need in the art for a fabrication method that reliably produces submicron gap widths in thin film staggered-pole heads. The related unresolved problems and deficiencies are clearly felt in the art and are solved by this invention in the manner described below.